Multi-carrier transmission methods are generally known as transmission methods with excellent resistance in a multipath environment. Because of these excellent characteristics, OFDM (Orthogonal Frequency Division Multiplexing), a multi-carrier transmission method which transmits over a plurality of orthogonal carriers, is used in digital terrestrial broadcasting and in wireless LANs (Local Area Networks).
An explanation is now provided for a receiving system using the example of digital terrestrial broadcasting. Standards for digital terrestrial broadcasting that use OFDM include the Japanese ISDB-T (Integrated Services Digital Broadcasting for Terrestrial) or the European DVB-T (Digital Video Broadcasting-Terrestrial). These standards insert a pilot signal, called an SP (Scattered Pilot), which is modulated at a predetermined amplitude and phase, into the transmission signal as a reference signal. At the receiving end, a method for demodulating and equalizing the received signal with reference to the amplitude and phase of the SP signal is used.
The SP signal determined by the above-mentioned standard is inserted once every 12 carriers in the carrier direction (frequency direction), and once every four symbols in the symbol direction (time direction).
FIG. 6 shows the basic structure of a digital terrestrial broadcasting receiver, a receiving apparatus. The receiving apparatus 500 comprises an antenna 501, a tuner unit 502, a synchronizing unit 503, an FFT (Fast Fourier Transform) unit 504, an equalizing unit 509, an error correction unit 510, and a channel estimation unit 511.
The channel estimation unit 511 includes an SP extraction unit 505, a known signal unit 506, a division unit 507, and an interpolation unit 508.
Letting a transmission signal be X(l, k), the channel characteristics and the noise component affecting the transmission signal be H(l, k) and N(l, k) respectively, and the received signal for this transmission signal be Y(l, k), then the following relationship (Equation 1) between these elements holds.Y(l,k)=X(l,k)H(l,k)+N(l,k)  Equation 1
In this equation, l is a symbol number, and k is a carrier number.
Within the channel estimation unit 511, the SP extraction unit 505 extracts an SP signal Y(l, kp) from the OFDM signal converted into a signal in the frequency domain by the FFT unit 504.
The known signal unit 506 outputs a known signal X(l, kp) which indicates a predetermined amplitude, a predetermined phase, etc. for an SP signal known at the receiving end.
The division unit 507 estimates the channel characteristics H^(l, kp) affecting the known signal X(l, kp), as acquired from the known signal unit 506, by dividing the extracted SP signal Y(l, kp) by X(l, kp), as in Equation 2 below.
                                          H            ^                    ⁡                      (                          l              ,                              k                p                                      )                          =                                            Y              ⁡                              (                                  l                  ,                                      k                    p                                                  )                                                    X              ⁡                              (                                  l                  ,                                      k                    p                                                  )                                              =                                    H              ⁡                              (                                  l                  ,                                      k                    p                                                  )                                      +                                          N                ⁡                                  (                                      l                    ,                                          k                      p                                                        )                                                            X                ⁡                                  (                                      l                    ,                                          k                      p                                                        )                                                                                        Equation        ⁢                                  ⁢        2            
“H^” is written in the present text because of character code constraints, yet writing “^” above “H” as in Equation 2 is actually appropriate. The same is true for “X^” and “Y^” below.
The interpolation unit 508 estimates H^(l, kd), the channel characteristics for data carrier signals other than the SP signal, by interpolating H^(l, kp), the channel characteristics for this SP signal, in the symbol direction (time direction) and the frequency direction (carrier direction). Data after equalization are sought as in Equation 3 by dividing the data carrier signal by the estimated channel characteristics.{circumflex over (X)}(l,kd)=Y(l,kd)/Ĥ(l,kd)  Equation 3
If the noise component is small enough to be ignored, the second element in the right hand side of Equation 2 can be ignored. In this case, the channel characteristics H^(l, kp) for the SP signal can be sought nearly accurately and are equivalent to H(l, kp). In an actual receiving environment, however, a noise component is superimposed. Thus the effects of noise cannot be ignored, and an estimated error is included in the channel characteristics H^(l, kp).
Since the SP signal thus includes an estimated error, an estimated error also exists in the channel characteristics H^(l, kd) for the data carrier signal estimated by interpolation processing which uses the channel characteristics H^(l, kp) for the SP signal. Therefore, the estimation precision of the channel characteristics deteriorates. So as not to worsen receiver performance, then, it is preferable to remove the noise component which causes estimation error when estimating a channel.
With regards to this issue, Patent Document 1 (Japanese Unexamined Patent Publication No. 2005-527153) proposes a method for estimating a channel via adaptive processing that uses a principle such as a Wiener filter. FIG. 7 is a block diagram of the receiving apparatus in Patent Document 1.
As shown in FIG. 7, the receiving apparatus 600 comprises an antenna 601, a tuner unit 602, a synchronizing unit 603, an FFT unit 604, an equalizing unit 609, a channel estimation unit 605, an autocorrelation calculation unit 606, a tap coefficient calculation unit 607, and a filtering unit 608.
The channel estimation unit 605 seeks the channel characteristics for the pilot signal, and outputs said channel characteristics to the autocorrelation calculation unit 606. The autocorrelation calculation unit 606 calculates the autocorrelation function for the channel characteristics, and supplies the calculated autocorrelation value to the tap coefficient calculating unit. The tap coefficient calculation unit 607 determines the tap coefficients for the noise removal filter, in accordance with the autocorrelation value supplied by the autocorrelation calculation unit 606. The filtering unit 608 performs filtering processing on the channel characteristics provided by the channel estimation unit 605 using the tap coefficients output from the tap coefficient calculation unit 607 and outputs the filtered channel characteristics to the equalizing unit 609.